Google "Eurail ferries” results in this page. Superfast is the one that you can use with a pass. You will however still need to pay port tax, season surcharge and a cabin (supposing you want one).

Their web page about Eurail/Interrail passes and how to book should be here but it seems their website has a problem at this moment.

There are/were at least 10-12 different ferries IT-GR-but as for now due to covid many have been suspended for the time being. Even with discount another one on a shorter route may even be less costly in full price. Also note that there is a tendency for ferries now to only take people with cars and leave the foot-pax (as hey call it) out to find other means.

there are at least also a dozen or so general sites that promise to overview and even book you on all ferry-lines-use google. Use these to find details, then search for the specific line to book on and/or see the info re acceptance of passes. PLUS that again due to covid its often not yet possible to see all precise details for next summer-it may change 10-12 times before that.

However, I somehow recall/remember a note some time ago that the only ferry that would take IR-holders for ´free´ was suspended and/or merged with another one and this has since been not possible anymore. If anyone knows current, please advice!

Again; the site seat61 has details for all overland routings from UK to all EUR countries and thus incl. the link via IT to GR-but its not specific for pass-users. Its usually the best uptodate overiew though for what is running